Gas turbine engines, such as utilized with jet aircraft, are being designed with ever increasing performance requirements. One element of the engine which has been receiving attention is the seal created between the rapidly rotating blades and the surrounding casing. The combustion gases exiting the engine through the rotating blade system should be properly channeled and not be permitted to otherwise escape if efficiencies are to be maximized. It has been the practice to provide the blade tips with abrasive particles which scour the surface of an abradable material mounted in the surrounding casing in order to create a seal which prevents escape of the gases. The blades not only elongate during operation of the engine on account of temperature changes, but also move transverse to their axis of rotation as a result of aircraft operating conditions. Permitting the blade tips to scour the abradable material mounted in the casing allows a very tight dynamic seal to be formed.
Particulates have been applied to blade tips by various means, generally involving some sort of electro-deposition or sintering process. Neither of these processes, however, creates a fusion bond between the particulates and the blade tip. The particulates may become loosened from the tip during operation of the engine, with the result that engine efficiency may diminish over time.
Turbine blades and their tips, as well as various other gas turbine engine components, have recently been manufactured from various nickel based alloys. Attempts have been made to incorporate the particulates into these engine components through use of a laser beam. The nickel based alloys, however, may be precipitation hardenable alloys, so that resolidification subsequent to laser processing causes relatively large and undesirable cracks to be formed in the blade. Furthermore, the density of the particulates is relatively less than the density of the nickel based alloys into which the attempts have been made to physically incorporate the particles, and attempts to reduce the density differences in order to achieve a somewhat more uniform distribution of the particles have been reported. These methods do not, however, appear to be practicable, because of gross crack formation upon resolidification of the melt pool.
The present invention is directed to a method for applying abrasive particulates to a turbine engine blade tip through use of a laser beam. Coated particulates are mixed with a metal matrix, and both the particulates and the metal matrix are fine powders. The metal matrix material minimizes crack formation upon resolidification of the nickel base alloy of the turbine blade because of its content and because a relatively small melt pool is produced. The particulates are coated with a non-reactive material chosen from the metals of which the matrix is comprised and which forms a thermal barrier preventing the particulates from being melted during laser processing, and the surface of the thermal layer melts during processing in order to create a fusion bond with the matrix material which is itself fusion bonded to the substrate.